1. Field of the Invention
The present invention relates generally to the field of non-volatile memories, and specifically to a method and apparatus for reducing standby current and wake up time of redundant circuits in non-volatile memory devices.
2. Background Information
Non-volatile memory devices such as electrically erasable programmable read only memory ("EEPROM") or FLASH devices typically include an array of storage cells. In addition, such non-volatile memory devices include configurable storage cells, apart from the array of storage cells, which are programmed in the factory before shipping to customers. These programmable storage cells are sometimes referred to as content address memories ("CAMs"). The typical usage of CAMs include: (1) Storing addresses of bad rows and/or columns in the main array so that an incoming address to a bad row or column is diverted to a redundant row or column; (2) Storing trim setting for critical circuits that are required to be trimmed; and (3) Configuring various line items such as top or bottom boot for different customers.
One requirement is that the CAMs must have their outputs ready during read and standby conditions. This typically requires a gate voltage of greater than 2.7 volts (greater than the threshold voltage of the storage cell) and a drain voltage of 1 volt. One prior art technique involved using V.sub.CC for the gate and a bias generator circuit for generating the drain voltage. The bias circuit typically consumed about 2.5 micro amps (.mu.a) of current, which is a significant portion of the total standby current of 10 .mu.a. Since this current is high for standby, a prior art technique involved shutting down the bias circuit in deep power down mode. However, transitioning from deep power down mode requires turning on the bias circuit, which typically takes 600 nanoseconds. Moreover, since V.sub.cc was used for the CAM gates, the scheme did not work for lower V.sub.CC voltages such as 1.8 volts because the threshold voltage of a non-volatile storage cell is as high as 2.5 volts.